Critical behavior in light nuclear systems: Experimental aspects

An extensive experimental survey of the features of the disassembly of a\nsmall quasi-projectile system with $A \\sim$ 36, produced in the reactions of 47\nMeV/nucleon $^{40}$Ar + $^{27}$Al, $^{48}$Ti and $^{58}$Ni, has been carried\nout. Nuclei in the excitation energy range of 1-9 MeV/u have been investigated\nemploying a new method to reconstruct the quasi-projectile source. At an\nexcitation energy $\\sim$ 5.6 MeV/nucleon many observables indicate the presence\nof maximal fluctuations in the de-excitation processes. The fragment\ntopological structure shows that the rank sorted fragments obey Zipf's law at\nthe point of largest fluctuations providing another indication of a liquid gas\nphase transition. The caloric curve for this system shows a monotonic increase\nof temperature with excitation energy and no apparent plateau. The temperature\nat the point of maximal fluctuations is $8.3 \\pm 0.5$ MeV. Taking this\ntemperature as the critical temperature and employing the caloric curve\ninformation we have extracted the critical exponents $\\beta$, $\\gamma$ and\n$\\sigma$ from the data. Their values are also consistent with the values of the\nuniversality class of the liquid gas phase transition. Taken together, this\nbody of evidence strongly suggests a phase change in an equilibrated mesoscopic\nsystem at, or extremely close to, the critical point.\n Comment: Physical Review C, in press; some discussions about the validity of\n excitation energy in peripheral collisions have been added; 24 pages and 32\n figures; longer abstract in the preprint